Recently, pen input devices have become available that electronically capture pen strokes on a surface (hereinafter referred to as “writing operations”) by sensing the time-dependent position of the pen on the surface and converting the pen positions to writing operations. However, traditional incremental movement detection techniques, such as those used in optical mice, fail when the pen input device is lifted off of the surface. Therefore, position detection techniques have been developed to determine the actual location (position) of the pen input device on the surface.
One position detection technique uses paper pre-treated with optically recognizable fiducials that allow the pen input device to determine its position relative to the fiducials. For example, as described in U.S. Pat. No. 6,650,320, a laser mounted on a digital pen input device directs coherent light towards special bar-coded paper. The reflected interference patterns are imaged and processed to determine the position of the pen input device relative to the known bar code patterns. However, requiring special paper increases the cost and limits the usage of such pen input devices.
Another position detection technique uses passive optical fiducials to determine the position of the pen input device. Typically, the pen input device is provided with wide field of view optics (e.g., panoramic optics) that enable an image of the surroundings of the pen input device to be taken. As long as a sufficient portion of the paper and its surroundings (e.g., three edges of the paper) are visible in the image, the position of the pen input device, along with the orientation of the pen input, can be computed. As writing operations are performed, the information written on the paper during the writing operations serve as additional fiducials.
However, the image processing involved in determining the position of the pen input device using passive optical fiducials is complex and computationally intensive. Additionally, frame rates in excess of 100 Hz are required to track the movement of the pen input device. In addition, some of the movement of the pen input device can be attributed to involuntary movements of the user's hand that result in no significant movement of the writing tip of the pen input device. Performing complex computations merely to determine that no significant movement occurred at the writing tip of the pen is inefficient and undesirable, especially for wireless, battery-powered pen input devices.
Some position detection techniques use the edges of the paper as references with respect to which the position of the pen input device is determined. However, under some circumstances, there may not be enough optical contrast between a blank sheet of paper and the background to enable the position of the pen input device to be reliably determined.
Therefore, what is needed is a technique for determining the path of a pen input device on a writing surface with reduced computational complexity and that does not require special paper.